Low RPM stutter and wobbly shifter

[MWS]

Originally Posted by marsheng

Plugs have the easiest time at low RPM. Although the CR may be quoted at 11:1 at low throttle openings the actual CR is much lower. Plugs fail at higher RPM and high throttle openings when the actual CR is higher and therefor more compressed air/fuel between the plug gap making it harder to jump the gap.

This is one reason I love this forum...being wrong, for only then do I learn something.

I should state that I'm not a mechanic (more of a home tinkerer), and the opinion I offered was based on other combustion engines (lawn mowers, etc) I've worked with that have a stutter at idle and I've found dirty/fouled plugs to be an issue. I'm not doubting your statement, but it would seem to me that the "misfire rate" at low RPMs would make a stutter much more pronounced, versus at a high RPM where the occasional misfire wouldn't be as detectable, or at least audibly. Again, I actually enjoy being wrong, so if you would be so kind as to offer a "Googleable" (LOL) starting point, I can start my education.

Actually (after re-re-rereading your post), it start to makes sense regarding density and gap as related to plug failure (or at least firing), but to me it still seems that once a plug had started the failure process that it would be most detectable at low RPM even though the CR is much lower. Grrrrrr.

Also, FWIW, (as stated in a previous post) I've also had an issue with a vehicle that had a misfire that plug leads were responsible for....changing them out solved my issue.

[marsheng]

A bit of 2 stroke history.

In the old days of 2 stroke and points, plugs had a real hard time. At low power settings, the plug electrodes did not get hot enough to burn off the oil/carbon on the electrode. Lawn mowers/bikes ran great. Next time you tryed to start it, nothing. The problem with points is that the spark rise time was slow. So slow that as the voltage rose, it would leak down through the carbon on the electrode and never jump the gap. Now take your neighbour that ran his mower full taps. Ie hot plugs. He never had a problem starting as his plugs were hot enough to burn off the carbon. The correct electrode/porcelain should look white to about 1/3 of its length. Ask any 2 stroke racer and he will tell you.

Now roll on the CDI - Capacitive discharge ignition. The rise time of the spark is very very high. So high that it will relieve its energy where ever it can. Down the electrode carbon as well as the spark gap so now the old fowled plug will still fire and in most cases the motor will run. Once it warms up the situation gets better.

I had a mini may years ago (a real one) and I made a CDI for it. One misty morning I started it and while running I heard this loud Clac Calc. I opened the bonnet. Due to the heavy mist the plugs were wet. No problem for this CDI , it had a nice fat blue spark running down the outside of the plug while the car was still idling perfectly.

TCI Transistor assisted ignition is a bit between the 2. Not sure what Porsche uses.

In most cases a good thrashing of the motor will burn of the carbon and return the plugs to normal.

PS if you always tootle around in your car, you need a hotter plug, if racing a colder one.

The operating temperature of a spark plug is the actual physical temperature at the tip of the spark plug within the running engine, normally between 500 and 800 °C (932 and 1,472 °F).

The heat range of the plug helps keep the tip at the correct temperature and burn of the carbon.

[Geof3]

Originally Posted by marsheng

In most cases a good thrashing of the motor will burn of the carbon and return the plugs to normal.

Which is one reason a Porsche motor (any really) usually runs better after a track day. And illustrates part of the point of not running these motors at low RPMS...